Re: [RFC 2/2] rcu: Remove ->dynticks_nmi_nesting from struct rcu_dynticks

[Joel Fernandes]

On Fri, Jun 22, 2018 at 02:16:00PM -0700, Paul E. McKenney wrote:
> On Fri, Jun 22, 2018 at 05:00:42PM -0400, Steven Rostedt wrote:
> > On Fri, 22 Jun 2018 13:58:13 -0700
> > "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > 
> > > Something like this:
> > > 
> > > 	IRQ entered
> > > 
> > > And never exited.  Ever.  I actually saw this in 2011.
> > 
> > I still believe this was actually a bug. And perhaps you made the RCU
> > code robust enough to handle this bug ;-)
> 
> Welcome to my world!
> 
> But I recall it being used in several places, so if it was a bug, it
> was an intentional bug.  Probably the worst kind.
> 
> Sort of like nested NMIs and interrupts within NMI handlers.  ;-)
> 
> > > Or something like this:
> > > 
> > > 	IRQ exited
> > > 
> > > Without a corresponding IRQ enter.
> > > 
> > > The current code handles both of these situations, at least assuming
> > > that the interrupt entry/exit happens during a non-idle period.
> > > 
> > > > > So why this function-call structure?  Well, you see, NMI handlers can
> > > > > take what appear to RCU to be normal interrupts...
> > > > > 
> > > > > (And I just added that fun fact to Requirements.html.)  
> > > > 
> > > > Yes, I'll definitely go through all the interrupt requirements in the doc and
> > > > thanks for referring me to it.  
> > > 
> > > My concern may well be obsolete.  It would be good if it was!  ;-)
> > 
> > I'd love to mandate that irq_enter() must be paired with irq_exit(). I
> > don't really see any rationale for it to be otherwise. If there is a
> > case, perhaps it needs to be fixed.
> 
> Given that the usermode helpers now look to be common code using
> workqueues, kthreads, and calls to do_execve(), it might well be that
> the days of half-interrupts are behind us.
> 
> But how to actually validate this?  My offer of adding a WARN_ON_ONCE()
> and waiting a few years still stands, but perhaps you have a better
> approach.

Hi Paul, I am Ok with adding a warning for a couple of releases if you and
others are Ok with it, how about something like this? Feel free to use the
diff as a starting point or a different approach if you/others prefer
something else. Thanks.

---8<-----------------------

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 8788ddbc0d13..176de74f5027 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -767,6 +767,21 @@ void rcu_idle_enter(void)
  */
 void rcu_user_enter(void)
 {
+	struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
+
+	/*
+	 * Add warning to ensure: no known instances of entering userspace from
+	 * IRQ/NMI handlers exist today. Currently special crowbarring of the
+	 * dynticks_nmi_nesting and maintaining of this separate counter when
+	 * dynticks_nesting exists, is done in order to handle entering of
+	 * userspace from an IRQ context and never returning. Lets track it for
+	 * a couple of kernel releases and then if the warning doesn't occur,
+	 * we can try to simplify the code and combine/eliminate the counters.
+	 * See: http://lkml.kernel.org/r/20180622181422.GT3593@xxxxxxxxxxxxxxxxxx
+	 */
+	WARN_ON_ONCE(rdtp->dynticks_nmi_nesting &&
+		     rdtp->dynticks_nmi_nesting != DYNTICK_IRQ_NONIDLE);
+
 	lockdep_assert_irqs_disabled();
 	rcu_eqs_enter(true);
 }